Heart valves play a pivotal role in circulatory function by maintaining the unidirectional flow of blood by opening and closing as a result of pressure differences on either side. However, natural heart valves may become dysfunctional from a variety of pathological causes such as stenosis and incompetence. A stenotic heart valve does not open fully due to stiffening of the valve tissue, thus more work is required for the heart to force blood through the valve. An incompetent valve causes inefficient blood circulation by permitting the flow of blood back into its originating chamber.
In many patients, a diseased heart valve can be replaced by a prosthetic heart valve. Prosthetic valves can be classified broadly into two principal types: mechanical and bioprosthetic. Mechanical valves are constructed exclusively from synthetic materials and are excellent in terms of durability. Traditional mechanical heart valves may produce good flow performance characteristics and potentially last longer than bioprosthetic valves, yet mechanical valves have a number of disadvantages. Mechanical heart valves require long-term or even lifetime anti-coagulation therapy to reduce the risk of thrombosis and embolism. Such a regimen effectively makes patients with mechanical heart valves borderline hemophiliacs. Patients with mechanical heart valves further require strict dietary and/or activity constraints, and the mechanical heart valve may produce an annoying valve “clicking” sound.
Bioprosthetic or biological valves include any valve that incorporates biological tissue, and themselves can be classified broadly into two principle types: the “graft-type,” in which substantially the entire valve is grafted from another individual; and the “tissue-type,” which are constructed in whole or in part with natural-tissue parts, such as valve leaflets. For the graft-type, an actual heart valve is retrieved from either a deceased human (homograft or allograft) or from a slaughtered pig or other mammal (xenograft). The retrieved valve can be preserved and/or sterilized, for example, homografts are typically cryopreserved and xenografts are typically cross-linked, typically in a glutaraldehyde solution.
Tissue-type bioprosthetic heart valves comprise assemblies having various amounts of biological material incorporated. Biological tissue typically is harvested from heart valves or from the pericardial sac of bovine (cattle), equine (horse), porcine (pig), or other mammalian sources. For example, some of these valves include leaflets derived from natural material (typically porcine) and still include the natural supporting structure or ring of the aortic wall. In other valves, leaflets derived from natural material (typically bovine pericardium) are trimmed and attached to a synthetic, roughly annular structure or ring that mimics the function of the natural aortic wall. In still other valves, both the leaflets and the annular support ring are formed of biopolymers such as collagen and/or elastin. All these valves, which include some biological tissue or biopolymers, are referred to herein as bioprosthetic valves, and include such assemblages as so-called “stented” valves which includes a stent and a biological valve member.
Bioprosthetic heart valves generally are less durable than mechanical valves, but they can alleviate some of the risks associated with mechanical valves, such as reducing the risk of forming blood clots, possible thrombosis and embolism, and/or the need for long-term anticoagulation therapy. Thus, problems related to the requirement for anticoagulants are usually short term with tissue-type valves and their failure is seldom abrupt. In addition, bioprosthetic heart valves are closer in design to the natural valve, have better hemodynamics, and do not cause damage to blood cells. However, biological heart valves are not without risk. Biological heart valves are susceptible to degeneration and/or calcification as a result of glutaraldehyde fixation, mechanical stresses, and the deposition of calcium phosphate on surfaces. Due to the degeneration of biological heart valves, such valves usually last about 10 to 15 years, often requiring additional surgery to replace or repair the valve.
Therefore, there exists a need for an improved biological or bioprosthetic heart valves that have good antithrombogenic, biocompatibility, and hemocompatibility properties. Such bioprosthetic heart valves should be less susceptible to degeneration and/or calcification and significantly improve the overall lifespan of the implant.